For instance, Patent Literature 1, as listed below, discloses a gas sensor which is equipped with two solid electrolyte bodies having oxygen ion conductivity and measures the concentration of NOx contained in exhaust gas from automotive vehicles. The solid electrolyte bodies are in the shape of sheet and face each other in a thickness-wise direction thereof. A space exists between the solid electrolyte bodies as a gas chamber into which gas (i.e., the exhaust gas) is admitted. Each of the solid electrolyte bodies has a surface exposed to the gas and an opposite surface exposed to a reference gas such as atmospheric air.
Each of the solid electrolyte bodies has electrodes formed on both the surfaces thereof. One of the solid electrolyte bodies (which will also be referred to as a first solid electrolyte body below) and the electrodes formed on the surfaces thereof constitute a pump cell. The other solid electrolyte body (which will also be referred to as a second solid electrolyte body below) and the electrodes formed on the surfaces thereof constitute a monitor cell and a sensor cell. The pump cell, the monitor cell, and the sensor cell are different in function from each other. These three cells are used to measure the concentration of a given gas component such as NOx contained in gas.
The above gas sensor is also equipped with a heater which works to heat the first and second solid electrolyte bodies up to an activatable temperature thereof. The heater is disposed so that it faces a surface of the first solid electrolyte body which is opposed to the surface thereof facing the second electrolyte body. A space exist between the heater and the first solid electrolyte body as a reference gas chamber into which the reference gas is admitted.